The real problem here is that I should not have to be logged in as root to run this script. (And also, running under sudo didn’t fix the problem anyway.)

So, if you are having this problem, take a look at your ~/.npm/ directory. Mine looked like this…

It looked like Yeoman was choking on modifying directories owned by root. Which of course is good behavior. What probably happened is I had installed some stuff at some point as root. Which is of course bad behavior